The present invention relates to a method and apparatus for tuning a lookup table circuit in a transmitter of an optical and electrical communication system. Communication systems are generally composed of a transmitter, a receiver, and a channel of which all may distort the signal. The transmitter generates a signal conveying information, which, after being transmitted over a channel, is received and recovered at the receiver. For optical based communications such as communications using vertical cavity surface emitting lasers (VCSELs), electro-absorption modulated lasers (EMLs), or directly modulated lasers (DMLs), the lasers are typically nonlinear laser sources. Further, distortion may be introduced in the electrical or optical transmission process because of reflections, linear and nonlinear channel impairments, or the like. When the nonlinear laser sources are used to transmit optical signals via a nonlinear channel to encode information in different amplitude levels (such as pulse amplitude modulation with 4 levels, “PAM-4”), the integrity of the communication can be compromised.
Lookup table circuits with configurable lookup table values are used in an optical transmitter to compensate for these nonlinearities. The lookup table circuits utilizes a suitable set of lookup table values to implement compensation into the optical transmitter. The look up table circuit may be a single lookup table or a combination of look up tables. The values of the look up tables may be mapped into the physical implementation of the lookup table circuit through firmware or software.
For the purpose of finding a suitable set of lookup table values, dedicated high cost measurement equipment having a high-speed sampling scope with pattern lock functionality or a real-time scope with large memory are commonly utilized. Such dedicated high cost measurement equipment can impose a heavy burden on the manufacturing process for producing optical transmitters. Therefore, a solution for finding a suitable set of lookup table values without using dedicated measurement equipment is preferred and needed.